1. Field of the Invention
The present invention relates to a method for making silicone rubber compositions which can be molded to produce electrically conductive articles such as electromagnetic interference (EMI) shielding. More particularly, the present invention relates to a method of blending curable silicone compositions with metal particulate, such as silver powder, to form injection moldable silicone blends capable of being stored for an indefinite period of time at ambient temperatures.
2. Background of the Invention
Conductive plastic materials comprising finely divided metal within a plastic matrix have been used to a considerable extent. For example, a heat- shrinkable article comprising a layer of finely divided metal filled plastic is described by Derby, U.S. Pat. No. 3,576,387. The metal filled plastic is used in combination with a heat shrinkable plastic to produce an electromagnetic shield for electrical conductors. Kwong, U.S. Pat. No. 4,695,404, is directed to conductive compositions consisting of a polymeric material filled with from 70 to 90 percent by weight of silver particles.
Cole et al U.S. Pat. Nos. 5,075,038 and 5,227,093 are directed to electrically conductive silicone compositions comprising a silicone polymer, or an organopolysiloxane convertible to a cured material by reaction with a curing agent, and an amount of silver particles sufficient to render the respective silicone compositions electrically conductive. Kroupa, U.S. Pat. No. 4,929,391 shows that electrically conductive silicone compositions can be made from polydiorganosiloxane and mixtures of silver and conductive carbon particles.
As shown by the art, various blending and molding procedures are available for making shaped articles such as electromagnetic interference shielding by incorporating a finely divided metal, such as silver, into a plastic or silicone medium. It has been found, however, it is often difficult to produce a molded metal filled plastic product where the metal is substantially uniformly distributed throughout the plastic matrix. As a result, the volume resistivity (milliohms-cm) of certain metal filled molded parts may lack uniformity. In addition, molding procedures for fabricating mixtures of plastic blended with large amounts of finely divided metal are often inadequate.
Efforts to improve the molding procedures of certain silicone curable mixtures are shown by Jeram et al U.S. Pat. No. 3,884,866 and Lee et al U.S. Pat. No. 4,032,502 which are both incorporated herein by reference. Lee et al, moreover, suggest that liquid injection molding can be used with certain silica filled curable silicone mixtures. In liquid injection molding, a two part mixture, for example, a mixture consisting of a Part A and Part B is often used. Part A can comprise a vinyl terminated polydiorganopolysiloxane fluid in combination with a platinum catalyst and an inhibitor. The curing agent, Part B, is generally a silicone fluid and may comprise a source of dimethylhydrogensiloxane units. The respective parts can be stored indefinitely at ambient temperatures. When mixed, however, the blend can be converted to an elastomeric article if injected into a hot mold.
It would be desirable therefor to provide a method for blending particulate silver and a heat curable silicone fluid to produce a substantially uniform mixture suitable for injection molding to fabricate conductive silicone parts. It also would be particularly significant if molded parts fabricated from such mixture of silicone fluid and finely divided metal exhibited a volume resistivity (milliohms-cm) value of about 2 or less.